The telescope was designed to gaze steadfastly at about 100,000 distant sun-like stars, searching for planets passing by, or transiting, relative to its line of sight. Detecting slight dips in the amount of light from a planet crossing the face of its parent star requires extremely precise pointing.

The telescope, the cornerstone of a US$650 million mission, lost that ability on Tuesday when a second steadying spinning wheel stopped working.

The telescope needs at least three of its four wheels operating to hunt for planets. It lost use of its first wheel last year.

“It certainly is not good news for the mission, which has been performing so well and had so much promise for doing even better,” deputy project manager Charles Sobeck, with NASA’s Ames Research Center, told reporters during a hastily arranged conference call on Wednesday.

The telescope is orbiting about 64 million kilometers from Earth, too far for a robotic or astronaut-led repair mission, added John Grunsfeld, NASA’s associate administrator for science.

“We’re not ready to call the mission over, but by any measure this has been a spectacularly successful mission,” Grunsfeld said.

In addition to trying to get the failed wheel working, engineers and scientists will be looking for alternative ways of operating the telescope.

They also will assess if Kepler could be used for other types of astronomical observations which do not require such precise pointing.

The goal of the mission is to find Earth-sized planets located the proper distance from their parent stars so they would have the right temperatures for liquid surface water. Water is believed to be necessary for life.

Though the telescope currently is not collecting any data, scientists have years of archived observations still to analyze.